1. Field of the Invention
The present invention relates to a sheet feeding apparatus for feeding sheets and a sheet processing apparatus for processing sheets.
2. Description of the Related Art
Conventionally, an image reading apparatus is known which reads images on a document in an image forming apparatus such as a digital copying machine or a facsimile apparatus. For reading images, in order to read image information on a document, an automatic document feeding apparatus (ADF: Auto Document Feeder, which may hereafter be referred to as ADF) is known which is a sheet feeding apparatus constructed to feed a document having a form of a sheet automatically to a reading position on a document base platen glass (refer to Japanese Patent Publication Laid-open No. 9-188420).
A conventionally known image reading apparatus is provided with two reading modes, i.e. an ADF reading mode in which the image on a document that is conveyed by the ADF placed above the document base platen glass is read by a stationary optical system and a book reading mode in which a document is set on the document base platen glass and the image on the document is read by moving the optical system.
An ADF has a document presence detection sensor that detects the presence or absence of a document on a document tray. For this reason, when a user places a document on the document tray, the document presence detection sensor detects that the document has been placed on the document tray.
Further, the ADF has a document length detection sensor that detects the length of a document (sheet) placed on the document tray. This document length detection sensor is constituted of a first document length sensor, a first document length sensor, a second document length sensor, and a second document length sensor. The first document length sensors and the second document length sensors are sensors of transmittance type similar to the document presence sensor. For this reason, when a document is placed on a document tray 1, the first document length sensors and the second document length sensors are moved by the load of the document (sheet), thereby shielding the light from the first document length sensors and the second document length sensors.
Here, when the light is shielded from only the first document length sensors, the apparatus interlocks with a width detection mechanism that detects with a width regulating plate to recognize that the document has a small size such as A4, A5, B5, LTR, or EXE. In contrast, when the light is shielded from both of the first document length sensors and the second document length sensors, the apparatus interlocks with the width detection mechanism to recognize that the document has a large size such as A3, B4, LGL, or LDR.
When there are no more documents placed on the document tray, each of the sensor flags placed in the ADF is returned to a predetermined position from the document detection position by an elastic member (not illustrated). Then, when the document presence sensor is brought into a detection-OFF state, the apparatus recognizes that there are no documents on the document tray.
However, in a conventional ADF construction such as described above, a user may commit erroneous placing in placing a document on a feeding tray of the ADF. Also, when the user has committed erroneous placing, the conventional ADF cannot issue a suitable error announcement to the user and, moreover, the ADF may execute an erroneous operation that is not intended by the user.
For example, there are cases in which the user has placed a document on the upstream side of the normal document placement position. This may bring the document presence sensor flag and the document presence sensor into a state in which the document cannot be detected in spite of the fact that the user has placed the document on the document tray. For this reason, the conveying document may not be brought into a permission (OK) state, or an error message stating the absence of the documents may be displayed.
Also, if the user presses a reading start button when the user has placed a document on the upstream side of the normal document placement position, the ADF may erroneously recognize that no documents have been placed on the feeding tray, thereby invoking an erroneous operation in which the ADF starts reading in a book mode for reading a document on a document base platen glass even though nothing is placed on the document base platen glass. At this time, when the feeding of the document is not started and the error message stating the absence of the document is displayed even though the document has been placed on the document tray, the user may consider that the apparatus is out of order without noticing the user's own placement mistake.
In recent years, despite the slow improvement of usability, the apparatus are increasing in their scale, complexity, and multifunctional property, so that an increasing number of users are not used to operation of the apparatus. For this reason, there is a fear that an increasing number of users consider that the apparatus is out of order without noticing their own placement mistake as described above.
In addition, there is a problem in that the apparatus starts to operate even if the document is placed by overriding the width regulating plate. This phenomenon occurs when the user places the document by letting the document override the width regulating plate in placing the document on the document tray. At this time, the front end of the document is pressed in to the normal position though the width of the document is not regulated, so that the document presence sensor flag and the document presence sensor (document presence detection mechanism) detect the presence of the document.
When this occurs, the document is started to be fed even though the document is not correctly placed to override the width regulating plate. When the document is conveyed in this state, the document may greatly meander, slip by receiving an unexpected resistance, or cause decrease in the conveying speed because no regulation has been made in the width direction. When this phenomenon intensely occurs, a jam of the document occurs, thereby raising a problem.
Also, when the placed document is a largely curled document or a document having a crease, there is a fear that a jam of the document may occur because the document cannot be conveyed due to the curl or crease after the document is fed.